PURPOSE: To provide a simple and inexpensive protocol converter in a short time and in a form accordant with a relevant purpose.

CONSTITUTION: A basic part consists of the input/output devices A1 and A4 which transfer the data between the equipments of different types which communicate with each other and the input/output programs A2, A3 and A5-A7 of lower ranks which directly control the devices A1 and A4. Then a protocol program main body A8 of a higher rank having an optical form is added together with a function routine group A9 of a medium rank which operates the input/output program in response to the request of the main body A8.